A surname from Welsh [in turn originating as a patronymic].

(mathematics) A method for proving Gödel's incompleteness theorems without the assumption that the theory being considered is ω-consistent. While Gödel's original proof uses a sentence that states (informally) "This sentence is not provable", Rosser's trick uses a formula that says "If this sentence is provable, there is a shorter proof of its negation".

(mathematics, computing theory) A theorem stating that, when applying reduction rules to terms in the lambda calculus, the ordering in which the reductions are chosen makes no difference to the eventual result.